Dissolution of blood clots (fibrinolysis) requires plasmin, a protease derived from the activation of plasminogen by tissue plasminogen activator (tPA). Both plasminogen and tPA are known to bind to the surface of platelets, where their interaction becomes greatly accelerated. Therefore, platelets are important promoters of fibrinolysis. Our laboratory has been examining the interaction of plasminogen and platelets with two types of assays: equilibrium binding experiments and rate measurements of platelet-dependent plasminogen activation. We have found that stimulation of platelets with thrombin increases high-affinity binding of plasminogen to the platelet surface and greatly accelerates the rate of plasminogen conversion to plasmin in the presence of platelets. Platelets that have been treated with carboxypeptidase B, which removes C-terminal lysines, lack high-affinity plasminogen binding and demonstrate much less ability to support plasminogen conversion. Plasminogen binding and conversion are also reduced by F(ab')2 fragments of anti-fibrinogen antibodies, but the effect is not as marked as the carboxypeptidase effect. Our goal is three-fold: (1) to determine the number and affinity of binding sites sensitive to carboxypeptidase and to anti-fibrinogen; (2) to establish whether these classes of sites are different from each other; and (3) to determine the relative importance of these classes of sites in promoting plasminogen conversion to plasmin.